Method of fabricating tubes for heat exchangers



March 18, 1958 M. w. ORR. JR, ETAL 2,827,551

METHOD OF FABRICATING TUBES FOR HEAT EXCHANGERS Filed Jan. 27, 1956 2 Sheets-Sheet l H l I INVENTORS: MALCOM W. ORR JR. [2 TYHOMAS L. MABERY 14m A. Low

ATTORNEY March 1958 M. w. ORR, JR, gym. 2,827,551

METHOD OF FABRICATING TUBES FOR HEAT zxcnmczns Filed Jan. 27, 1956 2 Sheets-Sheet 2 lmmmm' lmummll FIG. 8

lNVENTORs: MALCOM W. ORR JR. BEHOMAS L. MABERY [21m MOZMM ATTORNEY United States Patent M METHOD OF FABRICATING TUBES FOR HEAT EXCHANGERS Malcolm W. Orr, Jr., Signal Mountain, and Thomas L. Mabery, Chattanooga, Tenn., assignors to Combustion Engineering, Inc., New York, N. Y., a corporation of Delaware Application January 27, 1956, Serial No. 561,751

12 Claims. (Cl. 219-137) The present invention relates to the fabrication of tubes for use with heat exchangers and particularly to an improved method of fabricating tubes with laterally extending fins.

It is common practice in the art of boiler manufacture as well as manufacture of other heat exchange apparatus to employ what is termed peg finned tubes rather than tubes with a continuous fin extending longitudinally throughout the length of the tube. One such place where these peg finned tubes are often employed is in the water walls of the furnace of modern boilers. These peg fins comprise individual elements (pegs) which are welded to the tube and usually extend radially from the tube with these pegs generally being rectangular and arranged so that together the several pegs extend longitudinally throughout the length of the tube with individual pegs being slightly spaced from one another. With this arrangement, in the case of the water walled furnaces, the peg fins occupy substantially the complete space be tween adjacent tubes of the water wall with only the slight space between adjacent pegs then being unoccupied by the water wall surface.

The main purposes of employing this peg fin construction is to increase the surface of the fin relative to that provided with a continuous fin so that the heat transfor effectiveness of the fin is increased and also to prohibit destruction of or damage to the tubes as a result of excessive stresses that may build up in the fin. With respect to this latter purpose, the stresses that develop in a continuous fin both as a result of the welding operation and as a result of the rather high temperature difierential that exists between the distal end of the fin and the end that is welded to the tube cause a crack or break to develop in the fin which may extend through the fin and on into the tube. This result is avoided by the use of peg fins because the individual pegs are separate from one another and accordingly may move relative to one another so that excessive stresses are not created.

In fabricating peg finned tubes the so-called flash welding method has been employed wherein the tube forms one electrode and the peg the other electrode with the pegs being lightly placed against the tubes, generally from opposite sides, and a circuit then closed so that current passes across the joint. A high temperature is created because of the high electrical resistance developed at this joint which causes softening of the tube and peg after which the circuit is broken and the peg forced against the tube with a rather high pressure resulting in welding the peg to the tube. While this method is satisfactory it has certain disadvantages in that it requires that the tubes have a wall thickness sufficient to resist excessive deformation or burn-through during the welding operation and thereby often requires the use of a heavier wall than is necessary for the particular internal or external pressure with which the tube is to be used. This method is also relatively expensive in that 2,827,551 C Patented Mar. 18, 1958 the pegs must be specially formed for the welding machine and, to date, the pegs must be fed to the machine by hand since no suitable automatic feed is available although many attempts have been made to provide one. Furthermore, with this method the time required to peg a given length of tube is relatively slow and there is no known way to speed it up.

With the improved method of the present invention these disadvantages are overcome and a fin is provided on a tube in a most expeditious manner with the fin being so constructed and arranged as to provide an increased surface over a continuous fin and to insure against damage to the tube as a result of excessive stresses that may develop in the finned organization. In accordance with the present invention a tab-like fin means is bonded or welded to the tube by means of electric arc welding with the fin means being positioned against the tube and the electric are moved progressively along the juncture of the fin and tube to bond the two together. At certain predetermined frequent locations the bond is interrupted with the fin being free of the tube at these locations. Also at these locations the fin is weakened preferably by providing a slot therein which may extend entirely or part way through the fin. With this organization the portion of the fin means intermediate these locations may move relative to each other without causing damage to the tube Wall. If the slot extends entirely through the fin this relative movement will have no effect whasoever, while if the slot extends only part way through the fin this relative movement will cause a crack or break to develop in the remaining portion of the fin at this location. However, since the fin is not welded to the tube at this location this crack or break will not propagate into the tube wall and since a crack in the fin is of no importance whatsoever no damage will be done.

It is an object of this invention to provide an improved method of fabricating a finned tube. A further object of the invention is to provide such an improved method which insures against damage to the tube resulting from the development of excessive stresses in the fin organization during service. Another object of the invention is the provision of such an improved method which is expeditious and economical and provides for rapid finning of tubes.

The invention will be more fully understood from the following description when considered in conjunction with the accompanying drawing forming a part thereof and in which:

Figures 1, 2 and 3 diagrammatically represent the application of one form of the improved method of this invention whereby fins are welded to tubes with Fig. 1 showing the disposition of the fin means with relation to the tube immediately prior to the welding operation, while Fig. 2 shows these fin means after the welding operation, and, Fig. 3 is merely a perspective of the organization of Fig. 2. In this method depicted by Figs. 1, 2 and 3 the fin means is slotted as shown prior to the welding operation.

Figures 4, 5 and 6 indicate a somewhat modified form of the invention from that of Figs. 1, 2 and 3 with the fins being slotted after the welding operation rather than before the welding operation.

Figures 7 and 8 depict a further modified form with the slots being formed from the inner edge of the fin rather than the outer edge as in the case of the method depicted in Figs. 1 through 6.

Figures 9 and 10 indicate another modified form of the invention wherein the slots are formed by spacing individual members from each other longitudinally along the tubes. 1

. 3 a Referring now to the drawing and particularly to Figs. 1, 2 and 3, there is shown therein a tube such as may be employed in the water wall of the furnace of a steam generator or the like and which is to be provided with longitudinally disposed laterally extending fins. With the method depicted in Figs. 1, 2 and 3 the fins 12 are first provided with slots 14 which extend from the outer edge 16 of the fin inwardly a distance less than the full width of the fin. This results in forming tab-like members which are connected together at the root of the slot 14 and results in simplifying the handling operation over that which would be required if the tabs were entirely independent of one another. These fins are positioned against the tube 10 with the fins being diametrically opposed in the Fig. 1 showing. Thereafter the fins are welded to the tubes by means of electric arc welding with the arc being struck between the Welding electrode and the juncture of the fin and tube and progressively moved along this juncture at a rate properly, continuously welding the fin to the tube, with this being accomplished either by moving the welding electrode while the tube and fins are held stationary, by moving both the welding electrode and the tube and fins or by maintaining the welding electrode stationary and moving only the tube and fins.

At the location of each of the numerous slots 14 the welding electrode is moved away from the juncture of the tube and fin while the arc is maintained in existence with the electrode preferably being moved away from the tube and in parallel relation with the fin so that weld metal (symbolically illustrated on the drawing) is deposited upon the fin but the fin remains free of or unattached to the tube at these locations, with this result being shown clearly in Figs. 2 and 3. It will be noted that movement of the electrode away from the tube and out unto the fin at these locations causes weld metal to bridge the lower portion of the slots 14. This is of no particular consequence, however, and this weld metal may either be removed or left as is since the remaining portion of the slot will sufiiciently weaken the fin at these locations so that any crack that occurs in the fin because of the development of excessive stresses will be located at the root of the slot and will extend from the slot to the inner edge of the fin. Since the fin is not welded to the tube at these locations the crack will not propagate into the tube and so will have no harmful effect whatsoever.

The modified form of applicants invention depicted by Figs. 4, 5 and 6 is basically similar to that shown in Figs. 1, 2 and 3 except that rather than pre-slotting the fins prior to the welding operation the fins 18 are of the continuous or unslotted type as shown in Fig. 4 with these fins being welded to the tubes in the same manner as the fins 12 shown in Fig. 2 and with this welding operation being depicted and shown in Fig. 5 where it is shown that the welding electrode is moved away from the juncture of the fin and tube laterally out on the fin at numerous longitudinally spaced locations with the are being maintained in existence and weld metal deposited upon the fin 18 but the fin remaining unwelded or unsecured to the tube at these locations which are indicated in Fig. 5 by reference numeral 29. After the welding operation fins 18 are slotted by cutting, sawing, burning or any other manner which is sufficiently economical, with the slots being formed from the outer edge of the fin toward the inner edge of the fin at the location 2%. As shown in Fig. 6 these slots 22 do not extend entirely through the fin with this being entirely satisfactory for applicants purposes, although, of course, if desired the inwardly as was the case in the first described method. The welding operation is the same as that of the two previously described forms of applicants novel method with the result obtained by this operation being shown in Fig. 8. The slots 26 serve the purpose of controlling the cracking or breaking of the fins due to the development of excessive stresses and prevent damage to the tube since these slots will snfiiciently weaken the fins so that any cracking that occurs in the fins will extend from the root of the slot outward and since the fin is not welded to the tube at the location of the slots 26 there will be no damage to the tubes as a result of these cracks or breaks in the fin. However, if it is desired to increase the surface area of the fin or to make absolutely sure that there will be no cracks or breaks in the fin between the locations of these slots, the slots may be extended entirely through the fin after the welding operation thereby completely severing the fin section intermediate these slots from each other.

in the modified embodiment of applicants method shown in Figs. 9 and 10 slots 28 in the fin means are formed by longitudinally spacing individual tab members 29 from each other as shown in Fig. 9 and retaining the tab members in this position during the welding operation which operation is the same as that of the previously described forms of the invention and is shown in Fig. 10 wherein it is indicated that the welding electrode is moved away from the juncture of the fin means and tube at the location of slots 28 with the are being maintained in existance' and weld metal being deposited upon the fin means and away from the tube at these locations. Here again, as in the case of the form of applicants method shown by Figs. 1, 2 and 3, the Weld metal will bridge the slots 28 with this being of no consequence whatsoever, as explained hereinbefore.

It is thus seen that with applicants improved method a tab-like fin construction is provided with arc welding instead of the heretofore fiash or resistance welding being employed and with the fin means being sufiiciently weakened at numerous predetermined locations where the fin is not welded to the tube so that no tube damage is experienced in service because of the development of excessive stresses in the fin means.

With the improved method it is possible to more rapidly and more economically fabricate such fin tubes, eliminating the costly operation of making exceptionally formed pegs; allowing the use of thinner walled tubes or heavier fins than heretofore possible; permitting the use of finned metal in mill struck sizes and lengths; producing a weld that is better in looks and utility; eliminating the deformation of the inner wall of the tube that is produced with heretofore known methods; and providing greater freedom in the spacing of the tab members. Thus an improved product is achieved with a saving of both time and money.

It will be understood that the above description is intended for the purpose of illustration only and that modifications such as will occur to those skilled in the art are possible and are embraced Within the scope and spirit of the invention.

What we claim is:

1. In a tubular element having laterally extending tablike fin means the method of fabricating said element comprising bonding the fin means to the tube by moving a localized heat zone in the form of an electric arc along the juncture of the fin means and tube with said zone being of sufficient intensity to fuse the fin means and tube together as it moves along, discontinuing the bond at predetermined frequent locations along the fin means by discontinuing the application of said localized welding heat to the juncture of the fin means and tubes, and providing lateral slots in said fin means at said locations;

2. The method of claim 1 wherein the slots are provided in the fin means prior to the welding operation.

3. The method of claim 1 wherein the slots are provided in the fin means after the welding operation.

4. The method of claim 1 wherein said slots are formed by providing individual tab members to make up the tab like fin means and spacing these members from each other at said locations.

5. The method of welding tab like fin means to a tube comprising maintaining the fin means juxtaposed to the tube striking an electric arc between an electrode and the juncture of the fin means and tube, moving said are along said juncture at a rate to progressively fuse the fin means to the tube, moving the are away from said juncture for a short time at numerous predetermined locations while keeping it in existence and continuing to move it longitudinally of the fin means thereby leaving said fin means free of the tube at these locations, and weakening the fin means at these locations.

6. The method of claim 5 wherein in moving said are away from the juncture of the tin means and the tube it is moved away from the tube and is maintained in existence between the electrode and the fin means.

7. The method of claim 6 wherein said fin means is weakened by providing lateral slots therein.

8. The method of claim 7 wherein said slots are provided prior to the welding operation.

9. The method of claim 7 wherein said slots are provided subsequent to the welding operation.

10. In a fin means that is welded to a tube and is provided at certain locations with laterally extending slots for weakening purposes as well as increasing the surface area the method of welding the fin to the tube comprising striking an arc between a welding electrode and the juncture of fin and the tube thereby fusing said fin and tube together, moving said are along said juncture at a rate to progressively fuse the said fin and tube together and moving said are away from said juncture along one of the members being welded at said locations for a short distance longitudinally of the fin while maintaining the arc in existence between the electrode and said one member thereby leaving said fin free of the tube at said location.

11. The method of claim 10 wherein the arc is moved away from the tube at said locations and out onto the fin.

12. In a tube having extended surface the method of fabrication comprising providing laterally disposed slots extending partially throughout the Width of an elongated fin at predetermined locations therealong, positioning the fin with its base juxtaposed to the tube, bonding the fin to the tube for establishing good heat conductivity therebetween by striking an are between a welding electrode and the juncture of the fin and tube thereby fusing the fin and tube together, moving said are along said juncture at a rate to progressively fuse said fin and tube together and moving said are away from said juncture out on said fin at said predetermined locations while maintaining the arc in existence thereby leaving the fin free of the tube at said locations.

References Cited in the file of this patent UNITED STATES PATENTS 1,831,343 Caldwell Nov. 10, 1931 1,932,610 Tilley Oct. 31, 1933 2,384,919 Huber Sept. 18, 1945 2,682,598 Macoy June 29, 1954 

